1. Field of the Invention
The present invention relates to a perovskite ceramic having a high dielectric constant and small dependences of dielectric constant on the temperature and the electric field strength, which is favorably employed in a ceramic dielectric layer of a small-size ceramic capacitor having a large capacity and small dependences of electrostatic capacity on the temperature and the voltage, and a fabrication method thereof.
2. Background Art
A ceramic capacitor having a large capacity is desired to have small dependences of electrostatic capacity on the temperature and the voltage for facilitating designing of an electric power circuit and stabilizing the circuit characteristics. For this reason, a perovskite ceramic employed in a ceramic dielectric layer of a ceramic capacitor is desired to have a high dielectric constant and small dependences of dielectric constant on the temperature and the electric field strength. On the other hand, in order to attain a large capacity, a sharp peak of dielectric constant accompanying a phase transition of the perovskite ceramic is generally utilized. Thus, the large capacity ceramic capacitor has such a disadvantage that the dependences of dielectric constant on the temperature and the electric field strength may be higher as the dielectric constant is higher. For the purpose of obviating this the defect, intensive investigations have been made for improving the temperature dependence of dielectric constant, while not so many studies have been being made for improving the electric field strength dependence of dielectric constant.
FIG. 1 illustrates a process of preparing a ceramic which has heretofore been practiced for securing an improved dependence of dielectric constant on the temperature. Powders of raw materials are so weighed as to provide a composition ratio of a desired composition, to which a depressor agent is added for improving the temperature dependence of dielectric constant After wet-mixing and drying, calcination is made to effect crystallization of the composition, followed by crushing The crushed composition is subjected to pressing and sintering. Thus, a ceramic is prepared.
The above-mentioned addition of the depressor agent to the composition in the course of weighing dulls the peak of dielectric constant accompanying the phase transition of the crystals, thus providing a small temperature dependence of dielectric constant.
As an instance of an improvement of the temperature dependence of dielectric constant according to the above-mentioned method, Coffeen W. W. reported the results of addition of a metal stannate as the depressor agent to BaTiO.sub.3 as a perovskite ceramic material (see J. Amer. Ceram. Soc., Vol. 37, pages 480-489, 1954). According to Coffeen, when a small amount of Bi.sub.2 (SnO.sub.3).sub.3 is added to BaTiO.sub.3, the temperature dependence of dielectric constant is markedly improved, but the dielectric constant is decreased to about 1,000 or less. On the other hand, when 13 mol % of PbSnO.sub.3 is added to BaTiO.sub.3, the dielectric constant at room temperature is about 7,000, but the temperature dependence of dielectric constant is large. Okazaki et al. reported that, when NiSnO.sub.3 or La.sub.2 O.sub.3.3TiO.sub.2 is added as the depressor agent to BaTiO.sub.3, the ceramic turns to have a smaller temperature dependence of dielectric constant, and explained that the cause therefor may be attributed to formation of two phases in the ceramic (Journal of the Ceramic Society of Japan, Vol. 73, pages 106-112, 1965). However, in the case of the report of Okazaki et al., as low a dielectric constant as a mere 3,000 or less was obtained. Jonker, G. H. reported that two phases, a Ca-rich phase containing a large amount of CaTiO.sub.3 and a Ba-rich phase containing a large amount of BaTiO.sub.3, are formed in a ceramic of a binary system of BaTiO.sub.3 and CaTiO.sub.3, resulting in a smaller temperature dependence of dielectric constant (see Philips Tech. Rev., Vol. 117, pages 129-137, 1955). However, since the dielectric constant of CaTiO.sub.3 is as low as 130 at room temperature, the maximum value of dielectric constant in the binary ceramic after formation of the two phases is as low as 2,000 or less. Hennings, D. et al. reported that, when CdBi.sub.2 Nb.sub.2 O.sub.9 is added to BaTiO.sub.3, a ferroelectric phase and a paraelectric phase are formed in grain cores and grain shells, respectively, of crystalline grains, resulting in a smaller temperature dependence of dielectric constant (J. Amer. Ceram. Soc., Vol. 67, pages 249-254, 1984). In this case, however, the maximum value of dielectric constant is merely about 5,000. Particularly, in the case of a small temperature dependence of dielectric constant, the maximum value of dielectric constant is as low as about 3,000.
As described above, no perovskite ceramics having a high dielectric constant and small dependences of dielectric constant on the temperature and the electric field strength have heretofore been obtained.
Besides, the method comprising addition of a depressor agent requires a search for a specific depressor agent capable of providing a desired dependence of dielectric constant on the temperature in addition to a search for a perovskite ceramic material, and has disadvantages of liability to a decrease in resistivity, a shift of Curie temperature around which a high dielectric constant is exhibited, and/or a decrease in mechanical strength because of the addition of the depressor agent. Therefore, a difficulty has been encountered in controlling the temperature dependence of dielectric constant over a wide temperature range.